common emitter

Discussion in 'General Electronics Chat' started by muni, Aug 22, 2008.

  1. muni

    Thread Starter Active Member

    Jul 29, 2008
    45
    0
    sir, i've completed my AMIE degree through INSTITUTION OF ENGINEERS. Now i'm preparing for competitive exams.i request your help and guidance on my way.

    i request to clear my doubt with reasonable explanation.


    a common emitter transistor amplifier has a collector current of 1.0 mA when its base current is 25 micro ampere at the room temperature. it's input resistance is approximately equal to------------?



    the answer given inthe book is 1000 ohms
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The problem as you have stated it is missing a key parameter. That parameter is beta.

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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  4. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    give that Ic = 1ma and Ib = 25ua
    beta is roughly ic/ib so that makes beta 40

    input resistance is = 're * beta

    're is 25mv/IE = 25

    soooo input resistance = 1000

    checks out to me...
     
  5. muni

    Thread Starter Active Member

    Jul 29, 2008
    45
    0
    sir, thank you. the book has also given me same solution.but in the book after calculating beta value=40, it is given that

    for CE, Rm = (Vt(millivolts)(b+1))/1.025 milli amps

    here at this point i was unable to undersatnd.

    thank you for your solution sir.

    i,m preparing for INDIAN ENGINEERING SERVICES. I've so many such unanswered problems.

    sir i'm very new to this society. i don't know how to attach a file along with this quiery . i request you to suggest the way how i can add my files.

    my e mail address is psmuni@gmail.com
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Your book solution was just being more exact (pedantic) than S Lannan.

    The equations are the same.
    The 1.025 arises because Ie =Ic+Ib , Vt = 25 millivolts is an assumed standard value. the factor Beta+1 is more exact but most normally use just beta since beta >>1.

    For most purposes 'rule of thumb methods' are good enough.
     
  7. ruchimittal1986

    New Member

    Aug 23, 2008
    2
    0
    Hey Muni,
    Gr8, i am also very new to this site. Me also wants to prepare for IES. but though I am in job, i dont get much time for study..
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Doesn't the INSTITUTION OF INDIAN ENGINEERS teach basic electronics??
    No text book??
     
  9. muni

    Thread Starter Active Member

    Jul 29, 2008
    45
    0
    sir
    definitely IE teaches basics. but still some doubt arises due to lack of proper application.
    thank you for going through problem.
     
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